No brake pressure on front circuit after master cylinder replacement

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Originally Posted by 80stech

what was your bench bleeding procedure ? Do you have a kit with hoses to return to the reservoir?

Yes, I have that kit. When bench bleeding both circuits were pumping, so I suspect the seal gave way after it was installed on the vehicle. That could have been the "pop" I heard.

Originally Posted by 80stech

By "no pressure" you mean no air either? You were not able to "pump up" the brakes at all when bleeding?

No air either. Not able to pump up the brakes at all - pedal would sink 90% of the way to the floor at which point the rear circuit was pressurized by physical contact of the primary piston (look up how dual circuit master cylinders work when the primary circuit fails).

Originally Posted by 80stech

Is the front of the master cylinder also the front brakes ?

The front circuit is the primary circuit which is closest to the firewall.


New MC is on the way. I will not be bench bleeding this time just in case my bench bleeding procedure which pushes the piston all the way in (perhaps past normal pedal travel?) risks damaging the seals.

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Well guys, I'm losing my effing mind.

Removed the (new, recently-replaced) master cylinder. The front circuit seal had indeed failed completely - it would not build any pressure on the bench. Sent it back for a refund.

Got a replacement master cylinder. It built pressure on both circuits on the bench while bench bleeding, and I was very careful to not over depress the piston in case this could damage a seal. OK, so far so good. Installed it, thoroughly bled the system (pressure, DSC, and manual). Pedal feels firm and holds when vehicle is not running. When the vehicle is running (brake booster on) with firm pedal pressure the pedal will slowly sink 80% to the floor which is the same symptom I had previously where the front circuit seal had failed. Pumping the brakes firms things up, but the pedal will sink to 80% if you continue to press hard. I suspect that this 2nd new master cylinder has a leaking but not outright failed front seal.

Again, no bubbles, no leaks.

Unless someone has any insight, I'm going to order an OE MC from FCP Euro. If that works then I'm going to disassemble the Beck/Arnley MC from RockAuto to see what is wrong with the seal.